A bare bones basic package from me is $1200 a foot on the short wall for 6' and up with holding tanks. Everyone wants different things the last half dozen or so trailers I have done have all had soft walls and that is $500 extra, or $1000 with rope lights in the ceiling. I just finished a "Toter" that is an extended hood Peterbilt with a 14' box added to it, it had a 7' slide-out in it. Soft walls, real hardwood, about 40' of rope lights, all solid wood cabinets and trim. He added an electric awning, ducted air, and 2 flat screen tv's total cost was just under $18K. I'll be posting pictures as soon as the dinette gets recovered.

I'm not a conversion company, I just do this part time. But, it should give you an idea of cost you will be facing.